Parasitism in the garden is a common occurrence, though gardeners generally don’t pay much attention.
The most common observable parasitism is that of aphids and caterpillars by wasps. Affected aphids transform into round, tan colored mummies from which adult wasps emerge after utilizing the aphid’s body as room and board. Caterpillars are transformed into a nursery for hundreds of larval wasps which emerge to cover the caterpillar’s body with tiny white silk pupa cases.
Less observed is when host plants lose nutrients to parasitic plants. The degree of parasitism will vary by the dependence of the parasite on the host. Hemiparasitic plants, such as mistletoe, are capable of photosynthesis and only require water and nutrients from the host. Holoparasitic plants, such as the dodder, are completely dependent on the host for all of their growth needs. The host-parasite relationship rarely causes the death of either and is thought to be an unhappy balance.
The means by which parasitic plants extract nutrients and water from the host plant is through direct connection of the respective vascular systems. A small specialized appendage called a haustorium taps into the host vascular system. After seed germination and the start of plant growth, the parasite recognizes a host plants by chemical stimuli and responds by producing a haustorium which attaches to the host plant. From the haustorium, a penetration peg grows into the host tissue and establishes a vascular connection. Failure to find a host plant will result in death of the parasitic seedling.
In Hawaii, there are six known species of mistletoe, kaumahana or hulumoa. While they don’t resemble mistletoe depicted with Christmas and Druid celebrations, they do have similarities. The Hawaiian mistletoes grow on branches of large native hardwood trees, such as ohio, koa, akolea and lonomea, and are most often seen in forested areas. Hawaiian mistletoes have reduced leaf size, typically enlarged, flattened stems, and are said to resemble cactuses.
Several parasitic vines can be found growing in Hawaiian gardens. Cassytha filiformis, the laurel dodder, kaunaoa, pehu, kaunaoa malolo, kaunaoa uka, kaunaoa, malolo, pololo, love vine, or woe vine is a leafless vine green to orange in color. It parasitizes woody trees and shrubs and is a perennial plant. In many locations, Cassytha are found growing in the crown of trees. The dodder, Cuscuta sandwichiana, kaunaoa or kaunaoalei, is a native orange, stringy vine found growing near the ground around the state. Its host plants are mainly herbaceous annuals. In Hawaiian legend, the dodder is known as the motherless plant because it is a parasite. A closely related species, Cuscuta campestris, appears to be similar in appearance, however is a non-native from North America. Flowers of Cassytha are borne in small panicles or clusters and have fleshy seeds, while Cuscuta flowers are borne singly and the fruits are dry.
Sandalwood, Santalum spp., can be found growing in a number of gardens. Four species are recognized in Hawaii. They are iliahialoe, S. ellipticum, iliahi, aahi, aoa, laau ala, wahie ala, S. freycinetianum, S. haleakalae, and S. paniculatum. These all connect to their hosts, such as koa, aheahea and Chamaesyce hypericifolia, through root contact. Native grasses are also thought to be host plants.
Parasitic green alga is fairly common, especially those on the windward side. The parasite is a filamentous green alga, either Cephaleuros virescens or Cephaleuros parasiticus. You can find C. parasiticus as spots on guava leaves and fruits. It infects through the leaf and causes subsequent leaf death in the infection area. C. virescens can appear on the upper leaf surface of hundreds of plants as circular mounds, burnt-orange to brown or rust-colored, up to about 2 centimeters in diameter. Alga infest avocado trees, tea, coffee, coconut, cacao, breadfruit, magnolia and mango to name a few.
Russell Nagata is the Hawaii County administrator of the University of Hawaii at Manoa College of Tropical Agriculture uand Human Resources. He can be reached at email@example.com.